This project undertakes to use quantitative information derived from two ultrastructural techniques to examine several aspects of the membrane physiology of heart muscle. The ultrastructural techniques are (a) quantitative description of the distribution and surface density of membrane particles and other surface features in electron micrographs of freeze-fractured membrane replicas and (b) morphometry of membranes in transmission electron micrographs of thin sections. The aspects of cardiac membrane physiology to be examined include (1) measurement of the contributions to sarcolemmal area made by caveolae and sarcolemmal folds in papillary muscles and cardiac Purkinje fibers before and after passive stretch; (2) identification of the sarcolemmal membrane particles seen in freeze fracture replicas with permeability channels for ions in embryonic chick hearts (the identification is made by correlating the appearance and distribution of particles at different developmental stages with electrophysiological observations and with tetrodotoxin binding); (3) estimation of the sarcolemmal area per unit cell volume involved in nexal coupling at different developmental stages of embryonic rabbit ventricle and correlated freeze fracture studies of nexal membrane structure; and (4) measurement of cable properties, gap junctional (nexal) area, and gap junctional structure in cardiac Purkinje fibers under conditions known to cause electrical uncoupling of heart muscle cells.